Study On The Preferential Intercalation Behavior Of Clay In Immiscible PP/PS Blends

Polymer blends/clay nanocomposites have evoked great interest in both academic and industrial world, due to their excellent performance and potential application. Interestingly, preferential intercalation phenomenon of clay is present in polymer blends. The behavior of clay may play a significant role in the phase morphology and aggregate structure of polymer blends, and thereby influences the final performance of the composites. Based on the preferential intercalation phenomenon of clay in the polypropylene (PP)/polystyrene (PS) blends, the factors which control the preferential intercalation behavior are investigated. Moreover, the effects of the preferential intercalation behavior on the structure and properties of the composites are comprehensively studied.PP/PS/clay composites were prepared by melt blending at 160℃. The results show that the preferential intercalation phenomenon of clay does occur in the PP/PS blends, as evidenced by wide angle X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray energy dispersion spectroscopy (EDX). Independing on the proportion of PS and its feeding modes, PS chains intercalate preferentially into clay. Based on different dependence of melt viscosity on temperature between PP and PS, the effects of viscosity on the preferential intercalation behavior of clay are investigated via changing the processing temperatures. The clay platelets gradually transferred from PS phase to PP phase with increasing processing temperature. The results suggest that the preferential intercalation behavior of clay is determined by the difference between the viscosity of the two components. Furthermore, the higher viscosity of component, the stronger shear stress during the compounding process, consequently results in stronger capabilities of the intercalation.The polarity of polymer components was successfully changed through melt grafting maleic anhydride (MAH) onto PP chains and solution grafting sulfonic groups onto PS chains, respectively. To investigate the effects of polarities of components on the preferential intercalation behavior of clay, different PP/PS/clay based composites were prepared via melt blending. In the case of the two non-polar components, clay is totally dispersed in the PS domains. When PP (as the continuous phase) is polarized by introducing MAH groups onto PP chains, clay is preferred to disperse in PP phase. While for two polar components, the majorities of clay platelets are dispersed in PS domains, with partial ones locating at the interface. Infrared spectra (IR) and evaluation of the interaction energy density were employed to explain above phenomena. The results reveal that different interaction between clay platelets and polymer components leads to the preferential intercalation behavior. Most important of all, the polymer with high polarity generates higher interaction with clay than does that with low or/and no polarity, and hence results in stronger capability of preferential intercalation.Different PP/PS/clay based composites were fabricated by melt blending to study the effects of the preferential intercalation behavior of clay on the structure and properties of the composites. When clay is dispersed in the PS domains, the storage modulus of the composites is still enhanced, and the extent of increase in modulus is strongly depended on the dispersion of clay. Upon both of components intercalating into clay layers, the modulus of composites is further improved. It is worth noting that the composites exhibit the highest modulus as clay is dispersed in the continuous phase (PP), which is attributed to the restraint of clay platelets on the mobility of polymer chains. TGA results show that the better the clay disperses, the higher thermal stability of the composites could be achieved due to the better barrier effect of clay platelets. Moreover, the composite where clay preferentially resides in continuous phase has better thermal stability than that where clay locates in dispersed phase. The composite where the clay was intercalated both by PP and PS chains exhibits the highest thermal stability.Differential scanning calorimetry (DSC) was employed to investigate the effect of the preferential intercalation behavior of clay on the crystallization behavior of the composites. Under the isothermal crystallization condition, the clay platelets dispersed in domains do not influence the crystallization rate of the composites. The carbonyl group of the MAH can induce nucleation and the formation ofβcrystalline, resulting in the increase of the crystallization rate of the composites. Owing to the nucleating role of clay platelets, the presence of them in the continuous phase further accelerates the crystallization process of composites. The clay platelets at the interface show less nucleating effect. Under the nonisothermal crystallization condition, the preferential intercalation behavior of clay shows the similar effect on the crystallization behavior. However, the clay platelets dispersed in the domains can speed up the crystallization process, due to the increase of the viscosity of domains. The inducing behavior of MAH groups on the formation ofβcrystalline is weaker under nonisothermal condition than that under isothermal condition; meanwhile, the inducing behavior depends on the cooling rate.On the basis of the preferential intercalation behavior of clay, the compatibilization effects and mechanisms of clay on immiscible blends based on PP/PS were systematically investigated. The results show: (a) In the case of composite where clay preferentially disperses in continuous phase, the addition of clay reduces the domain size and improves the compatibility of blends, which can be explained by the fact that the clay platelets can effectively reduce the viscosity ratio of two components. (b) For composite where clay is intercalated by both components, the incorporation of clay dramatically reduces the domain size and plays a significant role in the compatibilization of polymer blends. The compatibilization behavior can be explained by what the 'in-situ grafts', formed at the interface due to the strong chemical interaction between components and clay platelets, reduce the interface tension and improve the interaction between the two components. (c) For the composite where clay is preferentially dispersed in the domains, the introduction of clay also can reduce the domain size and enhance the compatibility of blends. To explain the compatibilization mechanism, we proposed a model that clay platelets form the "clay knife" structure to split the dispersed PS domains.